Weather on M dwarfs to Y dwarfs

Cool stars 18 Splinter session proposal

Conveners: Sarah Casewell (University of Leicester) and Christiane Helling (University of St Andrews, UK)

There are over 1000 brown dwarfs that have discovered to date. With such a large sample, and the advent of large surveys (e.g UKIDSS, SDSS, WISE), it has started to become evident that some of these objects are variable.

While it is assumed most exoplanets have weather systems – either due to heating by their host star or winds circling them, it is only over the last decade  that we have really begun to detect variability in brown dwarfs.

Many brown dwarfs show periodic variability, which may be related to the object’s rotation. However, this variability may not be consistent across wavebands; potentially due uneven cloud structures, irradiation by a hot companion, or in a more extreme case, storms raging across the object's surface.  Multiwaveband observations allow us to probe the vertical cloud profiles in these atmospheres, and to study the atmospheric circulation in the brown dwarf regime in a way that is inaccessible for most hot Jupiter-like exoplanets.

Variability is also seen in some objects due to high energy processes - radio, H-alpha and X-ray emission are detected.  While the X-ray luminosity, as the classical high-energy indicator, decreases with decreasing mass from the M- into the Brown Dwarf regime, the radio emission prevails.  The physical reasons behind these observations have only started to be explored: synchrotron and auroral processes are discussed by various authors.  Whatever the nature of the underlying processes are, they all need an ensemble of electrons that is accelerated in a magnetic field. The necessary study of ionisation processes and of magnetic winds from ultra-cool atmospheric environments combines classical fields of stellar research with the new fields of planetary atmospheres and their dynamics.

This splinter session will to provide a platform to discuss observational and theoretical works related to weather (due to both high energy and atmospheric effects) in ultra-cool dwarf environments.

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